Metal Can Body

ABSTRACT

Can body including an essentially round cylindrical circumferential metal wall, wherein the wall has purposively selected thick and thin walled annular sections and the wall is at least partly provided beads.

The invention relates to a can body comprising a beaded, essentiallyround cylindrical circumferential metal wall.

Such a can body is known from e.g. EP0780314 disclosing a can-wall withbeads wherein various parameters defining the bead geometry aredisclosed. In particular in EP0780314 it is proposed to vary the beadlength depending on local susceptibility to collapsing.

According to the present invention it is an objective to improve canperformance by improving axial load and paneling properties and tothereby create opportunities to reduce consumption of packagingmaterial.

Axial load is to be understood as a load on the can wall caused byforcing the top towards the bottom of the can.

Paneling is to be understood as a phenomenon caused by forces acting onthe can wall where the forces are essentially not parallel to the wall,such as forces exerted on the can wall if the assembled en closed can isput in a pressurised vessel.

Although it is generally possible to increase either paneling strengthor axial load strength, in the known can to a certain extent the onegoes at the expense of the other.

According to the invention it is now possible to produce a can body inwhich a suitable combination of axial load strength and local panelingstrength can be achieved if the wall comprises purposively selectedthick and thin walled annular sections and the wall is at least partlyprovided with beads.

Until now, the extent to which one could use the effect of providingbeads to increase the can wall's strength in one aspect was limited bythe fact that the beads reduce the strength of the wall in anotheraspect. The inventors have realised that increasing the thickness of thematerial in the region where beading would be desirable could (morethan) compensate for the relevant strength reduction.

It was found that surprisingly, if a can body comprises a thickerannular wall portion in combination with beads according to a suitablebeading profile, technically feasible and improved combinations of axialand paneling strength as required in certain packaging cans such as foodcans, e.g. made by drawing and wall ironing (DWI), can be achieved, andconsequently better can performance and new down-gauging possibilitiescome into reach.

It should be noted that depending on the dimensions and properties ofthe packaging in question, the beads may be horizontal beads, i.e. beadsthat form an “endless groove”, the extreme of the “valley” lying in aplane perpendicular to the centre line of the can body, one or morespiral beads, or vertical beads.

As the bead profile in particular also the orientation of the beads,will influence the increase and/or reduction of the local axial strengthand paneling strength respectively, the effects of beading and“vary-thickness” can be varied and optimised taking also this factor of“shape and orientation of beading” into account.

It is remarked that providing thick and thin walled annular sections ina can wall of a drawn and wall ironed can body is known e.g. fromEP1294622 and U.S. Pat. No. 3,951,296. These publications concern canswith walls having at least one integrated reinforcing rib to achieveincreased resistance against buckling of the wall and improvedresistance against vacuum.

In a preferred embodiment the can body according to the invention is onewherein the metal can wall is wall ironed, such as in a DWI can. As itis well established to mass produce wall ironed cans and as it ispossible to perform wall ironing in such a way that a thicker annularwall portion is realised and considering that beading is a wellestablished method step in can-making, such a can body according to theinvention represents a very cost effective and reliable new packagingproduct.

In a preferred embodiment of the invention the can body is provided withat least an annular portion of a relatively small wall thicknessprovided with relatively shallow beads and an annular portion of arelatively large wall thickness provided with relatively deep beads soas to increase the ratio mechanical performance/metal consumption.

Mechanical performance should be understood as a combination of bothadequate axial load strength and adequate paneling strength meeting theapplicable requirements. Metal consumption should be understood as aterm that may be expressed in the form of a volume, thickness or weightof the sheet metal used for the making the can body in question and/orof the material forming part of the resulting can body.

By providing each annular portion of a can wall with locally optimisedcombinations of wall thickness and bead depth, it is possible to achievea lower packaging metal consumption for a certain “mechanical”performance, or conversely better performance for the same metalconsumption. This renders direct advantages of smaller materialconsumption, and further advantages regarding logistical andenvironmental aspects, e.g. in the form of reduction of weight to betransported and recycled in the distribution chain.

In an embodiment the annular portion of relatively large wall thicknessis positioned in the middle region of the wall. In essentiallysymmetrical packagings, e.g. food cans, the middle region of the wallwill generally be most susceptible to paneling. Providing the annularportion of relatively large wall thickness in the middle region of thewall, enables provision of (heavier) beading to the extent required witha view to optimising axial load strength and paneling strength locally.

In an embodiment there is an annular portion of relatively small wallthickness on either side of the annular portion of relatively large wallthickness. In a usual packaging, e.g. a known food can, the top andbottom region of the wall situated on either side of the middle regionare supported by the lid and the bottom of the closed can respectively.In this embodiment, according to the invention, the top and bottomregion are additionally supported by the annular portion of relativelylarge wall thickness positioned in the middle region of the wall. As aconsequence the top and bottom region become less critical regions forthe can performance aspects discussed here, and a smaller wall thicknesscan be realised there.

The invention will now be explained in more detail in non-limitativeexamples describing experiments that were conducted and results thatwere obtained.

Reference will be made also to the drawings showing in

FIG. 1 a schematic representation of the wall thickness at variouslocations of a known can body in unbeaded condition;

FIG. 2 a schematic representation of the wall thickness at variouslocation of a can body according to the invention in unbeaded condition;

FIG. 3 a known can bead profile and a can wall thickness profile;

FIG. 4 a can bead profile and a can wall thickness profile according tothe invention.

EXAMPLES

In order to establish the effects of the invention, two types of drawnand wall ironed cans were used in a set of trials.

One can type is a known standard Ø 73 mm 2 piece h0=110 mm drawn andwall ironed (DWI) beaded food can and the other can type is a can thatis very similar to the standard can in appearance and dimensions, buthas the features of the invention.

All cans were made from T57CA standard tinplate, using conventionalcan-making processes including drawing and wall ironing (DWI) andbeading.

The can body according to the invention was drawn and wall ironedaccording to EP1294622 using a stepped punch to realise a “drawn andwall ironed vary wall thickness can”. The wall of the can body wassubsequently provided with beads in a standard beading machine.

In order to carry out investigations regarding different bead profiles,the beading tools were built up using assemblies of individual beadingrings, allowing variation of the bead profile and bead groupings.

The key issue was to seek improved can concepts in view of standard foodcan requirements, namely regarding paneling: the closed can should beable to withstand a certain prescribed minimum pressure difference overthe can wall (external pressure less internal pressure) of e.g. 1.00, aswell as axial load strength: the closed can should be able to withstanda minimum axial load as defined hereinabove of e.g. 1500 N, by combininga “vary wall thickness” and a “wall bead concept”. The followingresearch programme of test runs was carried out:

TABLE 1 Test runs Thickness Bead profile Test Blank Thickness Nr beadsNr beads top run (mm) Can wall middle group bottom groups A 0.27 Vary 95 B 0.27 Vary 7 6 C 0.27 Vary 5 7 D 0.27 Vary 7 6 E 0.27 Vary 7 6 F¹0.27 Uniform 19 G² 0.27 Vary 19 H 0.26 Vary 7 6 ¹F represents a can bodyaccording to the state of the art that is manufactured in an industrialcan-making plant ²G is a copy of F with vary wall thickness,manufactured using lab can-making equipment

In the cans according to the invention, the thicker annular portion ofthe wall produced coincided with the middle bead group, the beadsrunning along the can wall being divided into 3 groups located in thetop, middle and bottom region of the can wall respectively, see e.g.FIG. 4.

Each beading group had a specific bead depth, e.g. for test run B thetop and bottom group (6 beads) 0.28 mm and 0.26 mm respectively, and themiddle group (7 beads) 0.37 mm, see FIG. 4

The resulting paneling and axial load properties follow from table 2below.

TABLE 2 Results Axial load Panelling Bead groups Test average Average(Nr. of beads) av. depth (mm) run (kN) (bar) Top-middle-bottom A 1.961.25 (5) 0.24 (9) 0.37 (5) 0.25 B 2.04 1.21 (6) 0.28 (7) 0.37 (6) 0.26 C2.08 1.16 (7) 0.20 (5) 0.35 (7) 0.24 D 1.99 1.23 (6) 0.24 (7) 0.40 (6)0.24 E 2.21 1.22 (6) 0.22 (7) 0.37 (6) 0.22 F 1.83 1.41 (19) 0.45 G 1.601.32 (19) 0.42 H 2.22 1.34 (6) 0.16 (7) 0.42 (6) 0.25

Large series of more than 10000 units were produced during trial runs.The results proved that the new and inventive can bodies allow apackaging steel gauge reduction from 0.27 mm to 0.26 mm, the resultingcan body according to the invention still complying with the applicablecan strength requirements, this representing an impressive improvement.

The axial load and paneling strength properties of the can bodymanufactured from down gauged packaging steel (test run H), viz. axialload 2.22 kN and paneling strength 1.34 bar, easily meet even thestrictest requirements. Results were fully reproducible and consistentthroughout the whole production in the trial run.

Test runs B, D and E produced satisfactory results and showed that amarked increase in axial and paneling performance is achieved in a varywall thickness and vary bead depth food can according to the invention.

Thanks to this, further down gauging of the packaging steel in questionto as thin as e.g. 0.255 mm and even thinner now becomes possible.

FIG. 1 in particular schematically shows the left half of a known canbody in unbeaded condition in longitudinal section. For the average canbody according to test run F, the wall at location h=1 mm had athickness of 159 μm, at h=5 mm a thickness of 160 μm, near the middle ofthe wall at h=50 mm the material had a thickness of 122 μm, the bottomof the can being located at h0=110 mm.

FIG. 2 in particular schematically shows the left half of a can bodyaccording to the invention in unbeaded condition in longitudinalsection. The average can body according to test runs A, B, C, D, E, G,at h=1 mm had a thickness of 140 μm, in the top region at h1=28 mm athickness of 113 μm, in the middle region at h2=54 mm a thickness of 138μm, in the bottom region at h3=76 mm a thickness of 106 μm, the bottomof the can being located at h0=110 mm.

In FIG. 3 the wavy line (black) most remote from the horizontal axismentioning h, represents the outer contour of a can body according totest run F. The horizontal axis represents h (in mm) as shown in FIG. 1.The vertical axis (dimension and scale not shown) represents the localcan radius: An area above the wavy line lies outside the can body.

In FIG. 3 the other line (grey) represents the thickness profile. Againthe horizontal axis represents h (in mm) as shown in FIG. 1. Here, thevertical axis (dimension and scale not shown) represents the localmaterial thickness.

As is shown graphically in FIG. 3, the known beaded can has a constantmaterial thickness over most of its can wall height, and a constant beadprofile over its beaded region, running from h=approx. 18 mm toh=approx. 87 mm.

In FIG. 4 the lines represent the same aspects as in FIG. 3, but now fora typical can body according to the invention, e.g. a can body accordingto test run E.

As can be seen in FIG. 4, in a preferred embodiment the can bodyaccording to the invention has in combination a stepped wall thickness(“vary wall thickness”) and bead groups. The wall thickness e.g. is inthe order of 110 μm from h=15 mm to h=35 mm, in the order of 140 μm fromh=45 mm to h=60 mm, and in the order of 110 μm from h=70 mm to h<110 mm.The respective annular thinner and thicker wall portions coincide withbead groups having beads with a smaller or larger bead depth.

For the beads located in the top and bottom regions where h is from 18mm to 38 mm and where h is from 68 mm to 88 mm, bead depth may be in theorder of e.g. 0.25 mm, and for those located in the middle region whereh is from 38 to 68 mm, bead depth may be in the order of e.g. 0.40 mm.

As a result of the invention, it is possible to increase panelingstrength where this is most desired, namely in the kind of cans underconsideration in the mid-height region, but without unacceptablyimpairing the axial strength, because additional axial strength isprovided by greater local material thickness.

It will be understood that for different can configurations with regardto bottom construction, manufacturing process and lid attachmentdifferent thickness and bead profiles may apply and that the resultingeffect of the combination of “vary thickness” and “vary bead profile”(such as in particular “vary bead depth”) may be optimised on a case bycase basis to find the ideal balance for product performance andmanufacturing effort in view of the specific packaging material to beused, e.g. packaging steel (tinplate) or aluminium sheet, polymer coatedsteel or aluminium sheet, the specific packaging variety, e.g. atwo-piece DWI packaging, and the purpose e.g. to realise a heattreatable packaging for preserved foods.

1. Can body comprising an essentially round cylindrical circumferentialmetal wall, wherein the wall comprises purposively selected thick andthin walled annular sections and the wall is at least partly providedwith beads.
 2. Can body according to claim 1, wherein the can wall iswall ironed.
 3. Can body according to claim 1, wherein the metal wall isprovided with at least an annular portion of a relatively small wallthickness provided with relatively shallow beads and an annular portionof a relatively large wall thickness provided with relatively deep beadsso as to optimise the ratio mechanical performance/metal consumption. 4.Can body according to claim 3, wherein the annular portion of relativelylarge wall thickness is positioned in a middle region of the wall. 5.Can body according to claim 3, wherein there is an annular portion ofrelatively small wall thickness on either side of the annular portion ofrelatively large wall thickness.